1. Field of the Invention
The invention relates to the field of determining the pulse transit time of a patient or donor where a pulse transit time is measured for pulse waves propagating via the patient""s or donor""s vascular system and created by his/her heart contractions.
2. Description of the Related Art
A patient""s or donor""s blood pressure is typically measured by means of an inflatable rubber cuff according to the Riva-Rocci method. This method allows a measurement only at a defined time, at which the pressure of the cuff is varied over a certain period of time. Thus, continuous measurement is limited to time intervals that are determined by the measuring method. A quasi continuous measurement would be associated with a constantly alternating expansion and deflation of the rubber cuff, which would be accompanied by unreasonable stress on the patient.
As an alternative to the non-invasive Riva-Rocci method, there exists a method for determining the pulse transit time, which can also be carried out non-invasively. This method is based on the knowledge that the time that a pulse wave, produced by a heart contraction of a patient or donor, requires to make its way through the vascular system from a first point to a second place is a function of the blood pressure of the person examined. If the time is measured that passes between the occurrence of a heartbeat (detected, for example, by means of an electrocardiograph (EKG)) and the time of arrival of the related pulse wave at an area of the body at a distance from the heart (detected, for example, by an optical sensor on the ear lobe or finger), this pulse transit time represents a direct measure of the patient""s or donor""s blood pressure. Since the pulse transit time varies from person to person, a calibration by means of an initial Riva-Rocci measurement is necessary. However, a statement on relative changes can be obtained directly from the relative changes in the pulse transit time. The relation between the blood pressure and the pulse transit time is largely linear (Psychophysiology, Vol. 3,86 (1976)). Since one measurement is possible per heart beat, this measuring method represents a semi-continuous blood pressure measurement.
The WO 89/08424 describes a measurement process for determining the pulse transit time by means of an electrocardiograph EKG and an optoelectronic measuring sensor on skin areas with good circulation. However, since the circulation in the skin tissue and thus also the photoelectric profile itself can change over time due to vasomotoric and other adjustments without the blood pressure necessarily having changed, a repeated recalibration should follow the initial calibration according to the Riva-Rocci method, using the measured values of the optoelectronic measuring sensor. In this respect, a constant relationship between the pulse transit time and the blood pressure is assumed for each person. The recalibration serves the purpose of allowing absolute statements about the systolic as well as the diastolic pressure from the photoelectric profiles at later points in time.
Acute emergencies, e.g. during hemodialysis and/or hemofiltration, require careful action. A primary complication during such a hemotherapy is a decrease in blood pressure. The most frequent cause of such an incident is a hypovolemia as a result of an excessively intensive fluid withdrawal. In particular during extracorporeal hemotherapy, it is, therefore, necessary to constantly monitor the blood pressure of a patient or donor in order to recognize possible circulation complications at an early stage.
The EP-A 0 911 044, which is hereby incorporated by reference, describes, among other things, a hemodialysis and/or hemofiltration apparatus, in which a continuous blood pressure monitoring with only a slight negative effect on the patient is made possible by means of a pulse transit time measurement. Using the measurement signal of the pulse transit time, it is possible to recognize critical blood pressure conditions at an early stage and to then inform the staff without delay. If necessary, countermeasures can be carried out automatically on the hemodialysis and/or hemofiltration apparatus, e.g. by infusions or modifying concentrations. This prior art apparatus, like the teaching of the WO 89/08424, assumes a constant relationship between the blood pressure and the pulse transit time. This assumption is not sufficiently accurate in the case of hemotherapies that It change the blood density in particular. In particular due to fluid withdrawal during a hemodialysis and/or hemofiltration treatment, the blood density increases during the course of the treatment (blood density in this case refers to the density of blood as a fluid per se). Since blood density has a direct influence on the pulse wave velocity and thus the pulse transit time, the results are inaccurate measurement values.
The present invention is based on the technical problem of improving a process and/or a device for determining a patient""s or donor""s pulse transit time in such a manner that the changes in the blood count are taken into account during the course of time and thus a more precise monitoring of blood pressure is made possible.
According to the teaching of the invention, this problem is solved by means of a process for determining the pulse transit time where a pulse transmit time is measured for pulse waves propagating via the patient""s or donor""s vascular system and created by his/her heart contractions, in which a value, correlating with the blood density, is determined and then used to calculate from the measured pulse transit time a pulse transit time, for which the influence of blood density is compensated.
The problem is also solved by a device for determining the pulse transit time with means for determining the pulse transit time of pulse waves, which are propagated via the patient""s or donor""s vascular system and are created by heart contractions, according to which there are means for determining a value, correlating with the blood density, and an evaluation unit that compensates for the influence of blood density on the pulse transit time.
The invention builds on the knowledge that the influence of a variable blood density between the two measurements can be compensated by means of measurements of a value, correlating with the blood density, at the time of a first pulse transit time measurement and at the time of a second pulse transit time measurement. In this manner a compensated first or second pulse transit time can be obtained that is directly comparable with the second or the first pulse transit time, as if it had been measured with constant blood density. In this manner, emergency conditions can be indicated with significantly greater reliability.
The rate at which a disturbance along an elastic, cylindrical, sufficiently long tube spreads in a homogenous fluid, may be expressed (Y. C. Fung, in xe2x80x9cBiomechanics Circulationxe2x80x9d, 2nd edition, Springer, N.Y., Berlin, 1997, p. 140):
c=[(A/xcfx81)(dp/dA)]xe2x80x83xe2x80x83(1)
where
xcfx81: density of the fluid
A: cross section of the tube
dA: change in cross section
dp: change in pressure in the tube
If equation (1) is assumed to be valid for blood in arteries, this results in equation (2) for blood with a density xcfx81(t0) at time t0 compared to blood with a density of xcfx81(t) at time t at constant blood pressure p(t0) for the pulse wave velocity c:
[c(t, p(t0), xcfx81(t))]/[c(t0, p(t0), xcfx81(t0))]=[xcfx81(t0)/xcfx81(t)]xe2x80x83xe2x80x83(2)
For the pulse transit time PTT, which indicates the passage of the pulse waves at the pulse wave propagation velocity over a defined path L, a similar expression is obtained:
PTT(t, p(t0), xcfx81(t))/PTT(t0, p(t0), xcfx81(t0))=L/c(t, p(t0), xcfx81(t))/L/c(t0), p(t0), xcfx81(t0))=[xcfx81(t)/xcfx81(t0)]xe2x80x83xe2x80x83(3)
By means of equation (3), it is possible to take into account the change in pulse transit time due to a change in blood density. For example, if at a time t0 a first pulse transit time PTT (t0,p(t0),xcfx81(t0)) was measured and at a second time t a second pulse transit time PTT (t,p(t),xcfx81(t)) was measured using equation (3), the influence of the different blood densities can be compensated. Each of the two pulse transit times can be converted to the blood density of the other measurement and thus made comparable:
PTT(t, p(t), xcfx81(t0))=PTT(t, p(t), xcfx81(t))[xcfx81(t0)/xcfx81(t)]xe2x80x83xe2x80x83(3a)
PTT(t0, p(t0), xcfx81(t))=PTT(t0, p(t0), xcfx81(t0))[xcfx81(t)/xcfx81(t0)]xe2x80x83xe2x80x83(3b)
The PTT data, compensated for the influence of blood density, can be directly compared and evaluated. If a calibration was carried out beforehand with an absolute blood measuring apparatus, the pulse transit time should be converted to the blood density at the time of the calibration measurements.
Thus, it continues to be possible to make a precise conversion into absolute blood pressure values.
The inventive process and/or the inventive device of the present invention embrace(s) this finding. In this respect it is sufficient to determine a value, correlating with the blood density, as long as the square roots in equations (3a) or (3b) can be determined for blood density compensation. The evaluation unit of the inventive device, which compensates for the influence of blood density on the pulse transit time, is suitable for carrying out a compensation, according to equations (3a) or (3b).
An especially preferred embodiment of the process, according to the invention, is used in an embodiment of the device, according to the invention, whereby the means for determining a value, correlating with the blood density, comprise a measuring device for determining the relative blood volume or the relative change in blood volume. Assuming that the change in density according to equation (3) was caused only by volumetric changes, but not by measurement changes, the following results for the root term from equation (3) with volumes V(t0) and V(t):
[xcfx81(t)/xcfx81(t0)]=[m/V(t)/m/V(t0)]=[V(t0)/V(t)]=[V(t0)/V0/V(t)/V0]=[RBV(t0)/RBV(t)]xe2x80x83xe2x80x83(4)
where V0 is a comparative volume for the relative blood volumes RBV. Thus, according to equation (4), it is sufficient to determine the relative change in blood volume; additional measurements for blood density or absolute data on blood volumes are not necessary.
In another embodiment of the invention, the means for determining a value correlating with the blood density are provided by a measuring apparatus for determining the hematocrit (HCT) and/or the relative change in hematocrit. If one assumes that during the measuring time, the number of red blood corpuscles and their size remain approximately constant, then the change in hematocrit is inversely proportional to the change in blood volume:
RBV(t0)/RBV(t)=HCT(t)/HCT(t0)xe2x80x83xe2x80x83(5)
Using equations (4) and (5), equation (3) can be easily converted into an expression in which, in addition to the pulse transit time measurement, it is then only necessary to indicate the relative change in hematocrit HCT(t)/HCT(t0).
Furthermore, the device according to the present invention exhibits advantageously as part of the evaluation unit an evaluation step that examines the values compensated according to equation (3a) or (3b) for abnormal values, using predefined criteria. For example, simple alarm threshold values can be set absolute or relative. The increase in pulse transit time over time t can also represent an alarm criterion. Lastly, if a calibration has been carried out with an absolute blood pressure measuring device, the pulse transit time can first be converted into an absolute blood pressure value and the alarm criteria can be applied to this value.
A preferred embodiment of the device, according to the invention, contains a unit for providing an EKG. The evaluation unit determines from the EKG the first reference point, ta, of the pulse transit time PTT. In addition, at a point at a distance from the heart, a unit for detecting the pulse waves is provided. The evaluation unit determines from the signal of this system the second reference point, te, of the pulse transit time PTT. In a preferred embodiment the detection unit is a photoplethysmograph. The pulse transit time PTT is shown as the interval between the two reference points (PTT=te-ta).
In a particularly advantageous embodiment, the means for determining the pulse transit time comprise at the same time the means for determining a value correlating with the blood density. Thus, for example, a photoplethysmograph can be used at the same time to determine the hematocrit.
The evaluation unit can also handle input and output functions with respect to the operating personnel as they are sufficiently well-known in the state of the art.
At this point it should be pointed out that the concept of the claimed invention can also be reapplied to the effect that the pulse transit time is not measured, but rather the pulse wave propagation velocity is directly measured. As evident from the equation (2), the dependency of the measurement values on the blood density can also be transferred directly to the pulse wave propagation velocity without diverging from the core idea of the invention. This case is regarded as an equivalent implementation of the invention.
The present invention is also directed to the problem of improving a hemotherapeutic arrangement with an extracorporeal blood circulation and a device for determining the pulse transit time of a patient or donor in such a manner that the changes in the blood count are taken into consideration over time and a more accurate monitoring of the pulse transit time and thereby of the blood pressure is thus made possible.
According to the invention, this problem is solved by a hemotherapeutic arrangement with an extracorporeal blood circulation and having a blood supply line connected at one end to the intake of the hemotherapeutic arrangement and at the other end for connection to the patient""s or donor""s vascular system, a blood removal line connected at one end to the outlet of the hemotherapeutic arrangement and at the other end for connection to the patient""s or donor""s vascular system, in which the arrangement has a device for determining the pulse transit time with means for determining the pulse transit time of pulse waves, which are propagated via the patient""s or donor""s vascular system and are created by heart contractions, means for determining a value correlating with the blood density, and an evaluation unit that compensates for the influence of the blood density on the pulse transit time.
As already stated above, in particular for extracorporeal hemotherapy, a constant observation of values, like the patient""s or donor""s blood pressure, is helpful. At the same time the blood count is automatically modified during hemotherapy. In particular, in the case of hemodialysis and/or hemofiltration, a change in blood volume also takes place. These forms of treatment, which are intended to replace the functions of the human kidney or at least supplement them, have the purpose, among other things, of controlling a patient""s fluid balance. At each treatment, a few liters of fluid are withdrawn from the patient during approximately 4-6 hours of treatment time. Hence there is a considerable change in blood density, even if fluid from other fluid compartments of the body flows in.
Integrating a device, for determining the pulse transmit time, as just summarized, into such a hemotherapeutic arrangement enables a continuous, precise blood pressure measurement. In addition, the actuator and sensor technology of the existing apparatus can be resorted to. As already described in EP-A 0 911 044, the unit for detecting the pulse waves at a point at a distance from the heart can comprise a measuring sensor that is already a part of the hemotherapeutic arrangement.
In an advantageous embodiment of the invention, this is the arterial pressure sensor, i.e., the pressure sensor that is attached to the blood supply line for a hemotherapeutic arrangement.
In the state of the art, there exist other sensors, by means of which blood volume and hematocrit changes can be determined extracorporeally. The EP-A 0 358 873 describes a system for determining the ultrasonic runtime that calculates from ultrasonic runtime the relative change in blood volume and/or hematocrit. There exist optical methods that determine with the optical direct light method the hematocrit concentration at the extracorporeal blood supply line, on the basis of which the hematocrit and relative blood volumes are derived. Such a process is the object of WO 94/27495, for example. A combination of an optical direct light method with a scattered light method, in which only a light wavelength needs to be used is proposed by WO 00/33053.
Other details and advantages of the invention are described in greater detail with reference to the embodiments illustrated in the drawings.